Relation between packing density and thermal transitions of alkyl chains on layered silicate and metal surfaces
Relation between packing density and thermal transitions of alkyl chains on layered silicate and metal surfaces
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DOI：
10.1021/la703019e
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发表时间：
2008-04-15
期刊：
影响因子：
3.9
通讯作者：
Farmer, B. L.
中科院分区：
文献类型：
--
作者：
Heinz, Hendrik;Vaia, R. A.;Farmer, B. L.
接枝到层状硅酸盐、金属和氧化物纳米颗粒表面的烷基链的自组装层被用来通过调节链在表面的堆积密度、头基官能度和链长度来控制与外部介质的相互作用。通过实验和模拟表征表明，烷基层的取向和加热时的可逆相变是烷基链的截面积与每个烷基链的有效表面积的函数。在均匀表面上，当堆积密度小于0.2时，烷基链在表面的取向接近平行，构象无序程度较高，没有可逆的熔融转变。堆积密度在0.2~0.75之间会导致中间倾角、半晶有序和加热时的可逆熔化转变。当堆积密度大于0.75时，表面活性剂在表面上几乎垂直排列，结晶度高，没有可逆的熔融转变。曲面可以通过相同的原理来理解，其中考虑了曲面上的局部曲率半径和与距离相关的堆积密度。在很好的近似下，这个简单的模型与烷基链的长度(至少需要C-10的长度才能形成足够独特的图案)、表面的化学性质和表面活性剂头基的化学性质无关。这些结构细节主要决定了烷基修饰表面的功能和热转变的温度。
Self-assembled layers of alkyl chains grafted onto the surfaces of layered silicates, metal, and oxidic nanoparticles are utilized to control interactions with external media by tuning the packing density of the chains on the surface, head group functionality, and chain length. Characterization through experiment and simulation shows that the orientation of the alkyl layers and reversible phase transitions on heating are a function of the cross-sectional area of the alkyl chains in relation to the available surface area per alkyl chain. On even surfaces, a packing density less than 0.2 leads to nearly parallel orientation of the alkyl chains on the surface, a high degree of conformational disorder, and no reversible melting transitions. A packing density between 0.2 and 0.75 leads to intermediate inclination angles, semicrystalline order, and reversible melting transitions on heating. A packing density above 0.75 results in nearly vertical alignment of the surfactants on the surface, a high degree of crystalline character, and absence of reversible melting transitions. Curved surfaces can be understood by the same principle, taking into account a local radius of curvature and a distance-dependent packing density on the Surface. In good approximation, this simple model is independent from the length of the alkyl chains (a minimum length of C-10 is required to form sufficiently distinctive patterns), the chemical nature of the surface, and of the surfactant head group. These structural details primarily determine the functionality of alkyl modified surfaces and the temperature of thermal transitions.